This invention relates to a filter means, and relates more particularly to tobacco smoke filter elements. More specifically, the instant inventive concepts are primarily concerned with filter means for cigarettes, although the products of this invention are generally useful in other filter applications, particularly for tobacco smoking means, whether they be cigarettes, cigars, pipes of the like. Since filters for cigarettes are particularly commercially important, the basic embodiments of the instant invention will be discussed as they relate to the production of filtered cigarettes.
In making tobacco smoke filters for use in connection with cigarettes and the like, bondable continuous filamentary tows of substantially continuous thermoplastic fibers, such as plasticized cellulose acetate fibers, polyethylene fibers, polypropylene fibers, nylon fibers and the like, have conventionally been employed as the starting material. The term "continuous filamentary tow", as used in this specification and the appending claims, is intended to define a material such as that which results when filaments extruded from a plurality of spinnerets are brought together and combined to form a continuous body of fibers randomly oriented primarily in a longitudinal direction. In such a tow, the filaments are generally longitudinally aligned in substantially parallel orientation, but include crimped portions which may form short section running more or less at random in non-parallel diverging and converging directions. Although the process of this invention is applicable to the various filamentary materials of this type, since plasticized cellulose acetate is the most common thermoplastic fiber used in the manufacture of cigarette filters, the specification hereof will be generally set forth in terms of this material. However, it is to be understood that the instant inventive concepts are not to be limited to this preferred embodiment.
In the manufacture of filters for cigarettes and the like, a number of different factors must be considered. Filtration efficiency, which is the capacity to remove unwanted constituents from smoke, while highly desirable is only one factor important in producing a commercially acceptable filter. Other factors, such as pressure drop, taste, hardness and cost also determine commercial acceptance of these products. For example, cellulose acetate, one of the most commonly used substances in manufacturing cigarette filters has a relatively low filtration efficiency. Increased filtration efficiency obtained by increasing the density or length of a cellulose acetate fiber may cause a pressure drop across the filter which is excessively and commercially unacceptable. The use of activated carbon or other such materials having higher filtration efficiency may increase cost and deleteriourly affect taste.
In recent years, air dilution has become a popular technique for compensating for the relatively low filtration efficiency of cigarette filters which have a pressure drop sufficiently low for commercial acceptance. In this technique, ventilating air is drawn into the filter peripherally and dilutes the smoke stream from the tobacco to thereby reduce the quantity of tar and other unwanted tobacco constituents drawn into the smoker's mouth with each puff.
The air dilution technique provides several obvious advantages:
It is an extremely economical method for reducing various solid phase constituents of tobacco smoke, generally referred to as "tar".
It also enables the removal or reduction of certain gas phase constituents of tobacco smoke such as carbon monoxide and nitrous oxide.
By varying the quantity of air introduced into the filter with each puff, it permits control, within reason, of the filtration process in order that efficiency and taste can be balanced.
One of the major challenges to the cigarette filter industry has been to design a filter and filter production techniques and apparatus for producing, at high speeds, large numbers of low cost filters capable of utilizing the air dilution technique. When the air dilution technique first became commercially important, most cigarette filters were produced with an over-wrap material applied to the outside of the filament bundle comprising the filter element in order to achieve a dimensionally stable product. The manufacturing process produced an axially elongated rod comprising a core of filaments contained by a surrounding over-wrap material called the "plug-wrap". After cutting the filter rods into small segments or plugs suitable for use as cigarette filters, a tipping over-wrap secured the segments to a tobacco column comprising a core of tobacco surrounded by a cigarette paper over-wrap. With the air dilution technique, cigarette filters produced in the foregoing manner required a porous or permeable plug-wrap in order that the air introduced generally through selectively provided perforations in the tipping over-wrap merged with and diluted to smoke coming from the tobacco column.
Because the use of plug wrap has certain disadvantages in general discussed in some detail in U.S. Pat. Nos. 3,313,306 and 3,377,220, granted Apr. 11, 1967 and Apr. 9, 1968, respectively, the subject matter of which are incorporated herein in their entirety by reference, techniques for producing non-wrapped dimensionally stable filter elements were developed. The significance of producing a non-wrapped, dimensionally stable filter rod is even more pronounced for use in air diluted cigarettes in view of the high cost of porous plug-wrap materials.
The techniques for producing a non-wrapped dimensionally stable filter rod disclosed in the aforementioned U.S. patents and related patents commonly assigned with the instant application, are highly useful and the best way presently known for such production. The filtering material which may be continuous filamentary tow, staple fibers or particulate in form is carried through the processing apparatus by an endless porous belt through which heated gas, such as steam, and coolant gas, such as air, are passed peripherally into the filtering material to bond the same into a dimensionally stable rod needing no plug-wrap at all. Thus, two advantages are gained from this very desirable method: elimination of the steps involved in applying the plug-wrap and elimination of the material costs of the plug-wrap itself, the latter being particularly significant when considering the high costs of porous plug-wrap necessary for use in an air-diluted filtered cigarette.
Although the endless belt method for formation of non-wrapped filters has recently been the subject of wide spread commercial interest, a number of disadvantages exist with respect to its use. The woven nature of the endless belt necessary to provide its porosity to steam and air embosses the surface of the filter rod with the pattern of the belt and leaves loose fibers which render the adhesion of the tipping paper to the filter element less efficient. The forming belt itself interferes with the step of steam penetration necessary to the formation of a dimensionally stable product and reduces the efficiency of the machine because the belts wear out and have to be changed, the belt-changing process resulting in down time for the filter rod production line.
Thus, the elimination of the belt for carrying the filtering material through the various processing stations would be desirable for obvious reasons. Yet, the belt was introduced, in part, as explained in the aforementioned patents, to eliminate the need to "pull" the filamentary tow, which is the most desirable commercial filtering material, through the system producing an undesirable tension on the individual filaments during processing, reducing the crimp initially present in the filaments and producing a rod with size, shape and functional characteristics which are difficult to control.
A highly desirable alternative to "pulling" the tow through the system or using a belt to "carry" the tow through the system would be to "push" the tow through the system. A pneumatic technique for making fibrous bodies has been disclosed in commonly assigned U.S. Pat. No. 3,313,665 granted Apr. 11, 1965, the subject matter of which is also incorporated herein in its entirety. In this technique, air or other feeding gas under pressure is used to "push" the tow through a confined zone where it is heated and cooled to bond the product into a rod. Moreover, this technique enables a reorientation of the individual fibers transversely of the longitudinal dimension of the rod, a feature to be discussed in more detail hereinafter. The process disclosed in U.S. Pat. No. 3,313,665 has found great commercial utility in the production of relatively large diameter "wicking" materials for felt-tip pends and the like, but has been completely unsuitable for the production of much smaller diameter cigarette filter elements because of the difficulty in dissipating the large volume of air necessary for propelling the fibrous material through the system shown in U.S. Pat. No. 3,313,665. Cigarette filter elements normally have a diameter of about 8 mm in contrast to "wicking" cartridges which are generally well over twice that size in diameter. When the confined zone is relatively large, as in the prior art process, the feeding gas may be substantially dissipated through a foraminous area of relatively short length due to the large circumference. Yet elimination of at least a major portion of the air prior to introduction of steam or the like is necessary to enable transverse penetration of the stem to the core of the tow for uniform bonding. Extension of the foraminous or porous zone, which is produced by a multiplicity of circumferential holes through the wall of the element defining the confined zone, is undesirable since such holes produce a rough inner surface which catch the tow and cause binding within the processing lines. Once the tow has been contacted with steam it shrinks slightly from the walls of the confined zone and is somewhat lubricated alleviating the foregoing problems. Therefore, it has been found that application of the pneumatic feeding technique to the production of small diameter rods such as cigarette filter rods can be accomplished if the amount of air can be reduced significantly from that required by prior art techniques to minimize the dissipation problem, a procedure which has not been possible heretofore, and/or if some of the air can be dissipated after steam introduction, a procedure which would result in non-uniform bonding at the core of the tow unless compensation is provided by additional residence time and significant transverse reorientation of the fibers in the presence of the steam prior to introducing cooling gas in contrast to the almost immediate cooling step of the prior art process. This enhanced residence time is particularly important in the high speed production lines necessary for commercial production of cigarette filter rods, generally well in excess of 75 meters/min and up to about 500 or more meters/min.
With respect to the high speed production requirements of commercial cigarette filter lines, it would also be highly desirable to produce a multiplicity of filter rods simultaneously from a single tow material. With prior techniques, as disclosed in the aforementioned patents, and others, such a procedure has been impossible since the resultant product was generally relatively rigid in its longitudinal dimension and could not be bent from its main direction of travel without damage, enabling only a single rod to be made in-line from a single starting tow. Producing a product having transverse flexibility would allow redirection of a plurality of rods formed from portions of a single tow for cutting the continuously formed rods into segments or predetermined length (which could be any desired multiple of a single filter element as is common in the industry). By producing a product having the individual fibers oriented in the rod generally in an adjacent and overlapping relation to one another in generally successive layers extending generally transverse to the longitudinal axis of the rod, limited transverse bending for subsequent processing would be possible and, additionally, the product would have a reduced resistance to flow of air in the transverse direction relative to its longitudinal resistance to flow, a property of which is very significant in enhancing the air-dilution properties of a cigarette filter.
Transverse orientation of fibers in a cigarette filter has been shown in commonly assigned U.S. Pat. No. 3,552,400, granted Jan. 4, 1971, the subject matter of which is also incorporated herein in its entirety by reference. However, such products are produced from staple fibers, not a continuous filamentary tow requiring over-wrapping and other attendant disadvantages such as difficulty in handling loose fibers and the like.
Transverse orientation of continuous filamentary tow fibers is disclosed in Japanese Patent Application No. 39-28359 (published Dec. 9, 1964), Japanese Patent No. 53-47599 (published Apr. 28, 1978) and European Patent Application No. 0018188 (published Oct. 29, 1980). The production techniques described in these documents require incoming tow velocity to withdraw filter rod velocity ratios of at least 6:1 and generally in excess of 10:1. This results in the incoming tow material having insufficient filling properties to cover the walls of the forming tube. Consequently, the material tends to form in one section, and results in a circular pattern around the forming tube. This yields a density stratification in the final product which is undesirable. More importantly, these techniques produce a product having an uneven surface to which a tip-wrap cannot be reliably bonded, particularly during high production speed application of the tip-wrap paper. This uneven surface apparently results from the fact that it is made up of successive radially-extending edges of the 180.degree.-bent continuous tow. In any case, reliable adhesion of the tip-wrap requires a coating or a plug-wrap to be applied to the filter, thereby increasing the expense of fabricating the product.